Log level should be optionally overridable on the individual Logger instances

[oakad]

Wouldn't it be great if instead of always relying on one global log level from core, it was possible to override the log level on each individual Logger instance? It is not uncommon for developers to want to focus on a particular part of their application and log level settable on an Logger can be of great help for that.

Loggers which don't have level override enabled will keep using the level from core remaining fully backward compatible.